This invention relates to the making of photolithography masks for semiconductor manufacturing and particularly to semiconductor technologies having minimum dimensions of 0.25 microns and less. More particularly, the invention relates to methods of proximity effect correction of photographic masks using a single component developer.
In the manufacture of semiconductor devices, the ever present need to continue increasing of the density of image segments continually causes the seeking of new and more efficient techniques for processing semiconductor devices. The manufacture of semiconductor masks is no exception and the move to sub-0.25 micron technologies is imminent.
The instant invention relates to chain scission copolymer photo resists of the type formed of 1:1 copolymerization of -chloromethacrylate and -methyl styrene, available as ZEP-nnn resists from Nippon Zen, Japan. These resists are sensitive to light and electron beam exposure which makes them particularly useful in mask manufacture. Various compositions of resist are available based on their molecular weight. For example, ZEP-520, having a molecular weight of about 50,000, or ZEP-7000, having a molecular weight of about 333,000. Once exposed, light sensitive portions of the copolymer are rendered soluble in organic solvents to produce good relief images. A typical application would to apply the resist in a solvent to a substrate, bake to remove the solvents, expose the resist via electron beam and then develop using a commercially supplied developer comprising one of the following developers, mixtures of diethyl ketone and diethyl malonate and single component solvents such as tolulene, xylene and alkyl esters of acetic acid such as amyl acetate or hexyl acetate. Following development, coated substrates are rinsed in a solvent such as 2-propanol.
The use of electron beams for lithography suffers from electron scattering in the substrate. This scattering is termed proximity effect. Many algorithms have been developed to correct for proximity effects. For raster scan electron beam lithography systems, GHOST proximity effect correction (PEC) is typically employed. GHOST PEC was developed in the early 1980""s (G. Owen, et al, xe2x80x9cProximity Effect Correction for Electron Beam Lithography by Equalization of Background Dose,xe2x80x9d J. Appl. Phys., 54(6), 1983). GHOST PEC consists of an additional exposure using the reverse polarity of the original pattern to be imaged into the substrate.
Typical problems with the use of GHOST correction are the increased process bias due to the additional GHOST exposure, the increased susceptibility to pinholes due to thinning of the resist film in the areas of the pattern where GHOST correction is applied, and potential degrade in critical dimension (CD) uniformity due to a reduction in resist contrast. To compensate for the increased process bias colder develop temperatures (16 deg C.) and shorter develop times have been employed (see xe2x80x9cA 180 nm mask fabrication process using ZEP 7000, multipass gray, GHOST, and dry etch for Mebes 5000xe2x80x9d, M. Lu, et al, SPIE Proc. 3376, Sept. 1998) using commercially available developers. In order to compensate for the additional resist loss in the areas which are processed using GHOST PEC, thicker resists are used (4000 ang used in previously quoted paper).
When previously practiced methods were attempted to be implemented in the manufacture of semiconductor masks of the sub-0.25 micron range, non uniform characteristics of the resulting photomasks were observed. This non uniformity can exist in both non-GHOST and GHOST processes. This was particularly true for the CD variation across a mask blank. In addition, for electron beam lithography forward scattering increases with resist thickness. So compensating for resist loss when undertaking GHOST PEC degrades resolution.
It has been discovered that the evaporative characteristics of the developer environment varied across the surface of the wafer, leading to non-uniform development due to varying concentration of reagents in the developer solution.
It is an object of the instant invention to provide an more uniform environment for the development of copolymer resists in order to provide more uniform and reproducible results in the critical dimensions in both non-GHOST and GHOST processes.
In accordance with the invention an active developer which comprises a substantially nonvolatile single component solvent is used for developing copolymer resists on semiconductor mask blanks. The preferred developer is ethyl 3-ethoxy propionate (EEP).
These and other objects of the invention will become more apparent when viewed in light of the following more particular description of the preferred embodiment of the invention.